Regulation of catalase activity and gene expression during Phytophthora nicotianae development and infection of tobacco

The catalase-peroxidase PiCP1 plays a critical role in abiotic stress resistance, pathogenicity and asexual structure development in Phytophthora infestans

Catalase-peroxidase is a heme oxidoreductase widely distributed in bacteria and lower eukaryotes. In this study, we identified a catalase-peroxidase PiCP1 (PITG_05579) in Phytophthora infestans. PiCP1 had catalase/peroxidase and secretion activities and was highly expressed in sporangia and upregulated in response to oxidative and heat stresses. Compared with wild type, PiCP1-silenced transformants (STs) had decreased catalase activity, reduced oxidant stress resistance and damped cell wall integrity. In contrast, PiCP1-overexpression transformants (OTs) demonstrated increased tolerance to abiotic stresses and induced the upregulation of PR genes in the host salicylic acid pathway. The high concentration of PiCP1 can also induced callose deposition in plant tissue. Importantly, both STs and OTs have severely reduced sporangia formation and zoospore releasing rate, but the sporangia germination rate and type varied depending on environmental conditions. Comparative sequence analyses show that catalase-peroxidases are broadly distributed and highly conserved among soil-borne plant parasitic oomycetes, but not in freshwater-inhabiting or strictly plants-inhabiting oomycetes. In addition, we found that silencing PiCP1 downregulated the expression of PiCAT2. These results revealed the important roles of PiCP1 in abiotic stress resistance, pathogenicity and in regulating asexual structure development in response to environmental change. Our findings provide new insights into catalase-peroxidase functions in eukaryotic pathogens.

An in-planta comparative study of Plasmopara viticola proteome reveals different infection strategies towards susceptible and Rpv3-mediated resistance hosts

Plasmopara viticola, an obligate biotrophic oomycete, is the causal agent of one of the most harmful grapevine diseases, downy mildew. Within this pathosystem, much information is gathered on the host, as characterization of pathogenicity and infection strategy of a biotrophic pathogen is quite challenging. Molecular insights into P. viticola development and pathogenicity are just beginning to be uncovered, mainly by transcriptomic studies. Plasmopara viticola proteome and secretome were only predicted based on transcriptome data. In this study, we have identified the in-planta proteome of P. viticola during infection of a susceptible ('Trincadeira') and a Rpv3-mediated resistance ('Regent') grapevine cultivar. Four hundred and twenty P. viticola proteins were identified on a label-free mass spectrometry-based approach of the apoplastic fluid of grapevine leaves. Overall, our study suggests that, in the compatible interaction, P. viticola manipulates salicylic-acid pathway and isoprenoid biosynthesis to enhance plant colonization. Furthermore, during the incompatible interaction, development-associated proteins increased while oxidoreductases protect P. viticola from ROS-associated plant defence mechanism. Up to our knowledge this is the first in-planta proteome characterization of this biotrophic pathogen, thus this study will open new insights into our understanding of this pathogen colonization strategy of both susceptible and Rpv3-mediated resistance grapevine genotypes.

Pathotypes of Fusarium oxysporum f. sp. fragariae express discrete repertoires of accessory genes and induce distinct host transcriptional responses during root infection

Convergent evolution of phytopathogenicity is poorly described, especially among multiple strains of a single microbial species. We investigated this phenomenon with genetically diverse isolates of Fusarium oxysporum f. sp. fragariae (Fof) that cause one of two syndromes: chlorosis and wilting (the 'yellows-fragariae' pathotype), or only wilting (the 'wilt-fragariae' pathotype). We challenged strawberry (Fragaria × ananassa) plants to root infection by five fungal isolates: three yellows-fragariae, one wilt-fragariae and one that is not pathogenic to strawberry. All Fof isolates had chromosome-level assemblies; three were newly generated. The two pathotypes triggered distinct host responses, especially among phytohormone-associated genes; yellows-fragariae isolates strongly induced jasmonic acid-associated genes, whereas the wilt-fragariae isolate primarily induced ethylene biosynthesis and signalling. The differentially expressed genes on fungal accessory chromosomes were almost entirely distinct between pathotypes. We identified an ~150 kbp 'pathogenicity island' that was horizontally transferred between wilt-fragariae strains. This predicted pathogenicity island was enriched with differentially expressed genes whose predicted functions were related to plant infection, and only one of these genes was also upregulated in planta by yellows-fragariae isolates. These results support the conclusion that wilt- and yellows-fragariae cause physiologically distinct syndromes by the expression of discrete repertoires of genes on accessory chromosomes.

Systemic Resistance Induction of Potato and Tobacco Plants against Potato Virus Y by Klebsiella oxytoca

Simple Summary

Klebsiella oxytoca, as a type of plant growth-promoting rhizobacteria (PGPR), was studied with regards to promoting plant growth and inducing plant systemic resistance against Potato Virus Y (PVY). The results of greenhouse experiments with tobacco and potato plants demonstrated that treatments with the Klebsiella oxytoca and biochar significantly enhanced the growth, while clearly lowering the disease severity and concentration of PVY. An RT-PCR analysis of the defense genes in the tobacco and potato treated with the Klebsiella oxytoca and biochar revealed an association with enhancing the systemic resistance of tobacco and potato to PVY. Klebsiella oxytoca and biochar may be considered valuable options to control PVY in potato and other Solanaceae crops.

Abstract

Potato Virus Y (PVY) is a serious potato disease that may significantly decrease potato production. To suppress potato virus infection, several measures have been undertaken. The utilization of plant growth-promoting rhizobacteria is one of these methods. Biochar soil treatment is believed to provide plants with a number of advantages, including increased plant growth and the development of systemic resistance to a variety of plant diseases. The goal of this research was to see whether adding biochar and Klebsiella oxytoca to the soil might cause PVY resistance and enhance the involved mechanisms in PVY resistance. Potato and tobacco seedlings treated with Klebsiella oxytoca and biochar exhibited the same impact of significant symptom reduction, with complete negative ELISA findings, supporting the antiviral activity of K. oxytoca and biochar. Furthermore, owing to the connection between the ISR implicated substrates, significant amounts of polyphenol oxidase, catalase, and superoxide dismutase were observed in treated plants, with the same behavior as defense genes expression levels. It may be a step forward in the development of biochar and K. oxytoca as potential environmentally friendly disease control strategies against PVY.

Impacts of Continued Exposure to a Susceptible Host Genotype on Aggressiveness of Phytophthora nicotianae Isolates Adapted to Multiple Sources of Partial Resistance

Pathogen adaptation can threaten the durability of partial resistance. Mixed plantings of susceptible and partially resistant varieties may prolong the effectiveness of partial resistance, but little is known about how continued exposure to a susceptible genotype can change the aggressiveness of pathogen isolates adapted to a source of partial resistance. The objective of this study was to examine the effects of continued exposure to a highly susceptible tobacco genotype on isolates of Phytophthora nicotianae that had been adapted to partial resistance. Isolates of P. nicotianae previously adapted to two sources of partial resistance were continually exposed to either the original host of adaptation or a susceptible host. After six generations of host exposure, isolates obtained from the partially resistant and the susceptible hosts were compared for their aggressiveness on the resistant host and for differences in expression of genes associated with pathogenicity and aggressiveness. Results suggested that exposure to the susceptible tobacco genotype reduced aggressiveness of isolates adapted to partial resistance in K 326 Wz/- but not of isolates adapted to partial resistance in Fla 301. Quantification of pathogenicity-associated gene expression using qRT-PCR suggested the rapid change in aggressiveness of isolates adapted to Wz-sourced partial resistance may have resulted from modification in gene expression in multiple genes.

First Report of Protective Activity of Paronychia argentea Extract against Tobacco Mosaic Virus Infection

The widespread use of chemical control agents and pesticides for plant-pathogen control has caused many human health and environmental issues. Plant extracts and biocontrol agents have robust antimicrobial activity against different plant pathogens. However, their antiviral activities are still being investigated. In the present study, the methanol extract of Paronychia argentea was characterized and evaluated for its protective activity against the tobacco mosaic virus (TMV) infection in tomato plants under greenhouse conditions at 21 days post-inoculation. The results showed that the foliar application of P. argentea extract (10 µg/mL) enhanced tomato plant growth, resulting in significant increases in shoot and root parameters and total chlorophyll contents. Moreover, a significant reduction in TMV accumulation level in P. argentea-treated plants of 77.88% compared to non-treated plants was reported. Furthermore, induction of systemic resistance with significant elevation in production of antioxidant enzymes (PPO, CAT, and SOD) and transcriptional levels of the pathogenesis-related proteins (PR-1 and PR-7) and polyphenolic genes (CHS and HQT) were also observed. Out of 16 detected compounds, HPLC analysis revealed that the most abundant polyphenolic compounds found in P. argentea extract were gallic acid (5.36 µg/mL), kaempferol (7.39 µg/mL), quercetin (7.44 µg/mL), ellagic acid (7.89 µg/mL), myricetin (8.36 µg/mL), and ferulic acid (8.69 µg/mL). The findings suggest that the use of P. argentea extract as an effective and safe source for the production of bioactive compounds may offer a solution for a promising approach for the management of plant viral infections. To the best of our knowledge, this is the first report of the protective activity of P. argentea extract against plant viral diseases.

Selective redox signaling shapes plant-pathogen interactions

A review of recent progress in understanding the mechanisms whereby plants utilize selective and reversible redox signaling to establish immunity.

Methyljasmonate and salicylic acid contribute to the control of Tilletia controversa Kühn, causal agent of wheat dwarf bunt

Tilletia controversa Kühn (TCK) is the causal agent of dwarf bunt of wheat, a destructive disease in wheat-growing regions of the world. The role of Meja, SA and Meja + SA were characterized for their control of TCK into roots, coleoptiles and anthers. The response of the defence genes PR-10a, Catalase, COI1-1, COII-2 and HRin1 was upregulated by Meja, SA and Meja + SA treatments, but Meja induced high level of expression compared to SA and Meja + SA at 1, 2, and 3 weeks in roots and coleoptiles, respectively. The severity of TCK effects in roots was greater at 1 week, but it decreased at 2 weeks in all treatments. We also investigated TCK hyphae proliferation into coleoptiles at 3 weeks and into anthers to determine whether hyphae move from the roots to the upper parts of the plants. The results showed that no hyphae were present in the coleoptiles and anthers of Meja-, SA- and Meja + SA-treated plants, while the hyphae were located on epidermal and sub-epidermal cells of anthers. In addition, the severity of hyphae increased with the passage of time as anthers matured. Bunted seeds were observed in the non-treated inoculated plants, while no disease symptoms were observed in the resistance of inducer treatments and control plants. Plant height was reduced after TCK infection compared to that of the treated inoculated and non-inoculated treatments. Together, these results suggested that Meja and SA display a distinct role in activation of defence genes in the roots and coleoptiles and that they eliminate the fungal pathogen movement to upper parts of the plants with the passage of time as the anthers mature.

Phytopathogenic oomycetes: a review focusing on Phytophthora cinnamomi and biotechnological approaches

The Phytophthora genus is composed, mainly, of plant pathogens. This genus belongs to the Oomycete class, also known as "pseudo-fungi", within the Chromista Kingdom. Phytophthora spp. is highlighted due to the significant plant diseases that they cause, which represents some of the most economically and cultural losses, such as European chestnut ink disease, which is caused by P. cinnamomi. Currently, there have been four genome assemblies placed at the National Center for Biotechnology Information (NCBI), although the progress to understand and elucidate the pathogenic process of P. cinnamomi by its genome is progressing slowly. In this review paper, we aim to report and discuss the recent findings related to P. cinnamomi and its genomic information. Our research is based on paper databases that reported probable functions to P. cinnamomi proteins using sequence alignments, bioinformatics, and biotechnology approaches. Some of these proteins studied have functions that are proposed to be involved in the asexual sporulation and zoosporogenesis leading to the host colonization and consequently associated with pathogenicity. Some remarkable genes and proteins discussed here are related to oospore development, inhibition of sporangium formation and cleavage, inhibition of flagellar assembly, blockage of cyst germination and hyphal extension, and biofilm proteins. Lastly, we report some biotechnological approaches using biological control, studies with genome sequencing of P. cinnamomi resistant plants, and gene silencing through RNA interference (iRNA).

Hydrogen peroxide generated by over-expression of cytosolic superoxide dismutase in transgenic plums enhances bacterial canker resistance and modulates plant defence responses

H₂O₂ generated during the oxidative burst, plays important roles in plant defenses responses against pathogens. In this study we examined the role of H₂O₂ on bacterial canker resistance in transgenic plums over-expressing cytosolic superoxide dismutase. Three transgenic lines (C64, C66 and F12) with elevated levels of H₂O₂ accumulation showed enhanced resistance against bacterial canker disease caused by Pseudomonas syringae pv. syringae, when compared to the non-transformed control. Analysis of the expression of several genes involved in the plant-pathogen interaction showed that the expression of those involved in SA pathway (pr1 and npr1) and JA (lox3) were activated earlier and transiently in transgenic lines C66 and F12 when compared to the wild type. However, the expression of genes involved in anthocyanin synthesis (chi, chs, f3h, dfr, atcs, myb10) and ethylene (acs) was induced at very low levels whereas it was activated by the pathogen at exaggerated levels in the non-transformed line. These results suggest that resistance observed in transgenic lines over-producing H₂O₂ is correlated with an early and transient induction of defense genes associated with the SA and JA pathways and inhibition of gene expression associated with ethylene and anthocyanin biosynthesis.

A proper PiCAT2 level is critical for sporulation, sporangium function, and pathogenicity of Phytophthora infestans

Catalase is present in prokaryotic and eukaryotic organisms and is important for the protective effects of the antioxidant system against free radicals. Many studies have confirmed that catalase is required for the growth, development, and pathogenesis of bacteria, plants, animals, and fungi. However, there has been relatively little research on the catalases in oomycetes, which form an important group of fungus-like eukaryotes that produce zoosporangia. In this study, we detected two Phytophthora infestans genes encoding catalases, but only PiCAT2 exhibited catalase activity in the sporulation stage and was highly produced during asexual reproduction and in the late infection stage. Compared with the wild-type strain, the PiCAT2-silenced P. infestans transformants were more sensitive to abiotic stress, were less pathogenic, and had a lower colony expansion rate and lower PiMPK7, PiVPS1, and PiGPG1 expression levels. In contrast, the PiCAT2-overexpressed transformants were slightly less sensitive to abiotic stress. Interestingly, increasing and decreasing PiCAT2 expression from the normal level inhibited sporulation, germination, and infectivity, and down-regulated PiCdc14 expression, but up-regulated PiSDA1 expression. These results suggest that PiCAT2 is required for P. infestans mycelial growth, asexual reproduction, abiotic stress tolerance, and pathogenicity. However, a proper PiCAT2 level is critical for the formation and normal function of sporangia. Furthermore, PiCAT2 affects P. infestans sporangial formation and function, pathogenicity, and abiotic stress tolerance by regulating the expression of cell cycle-related genes (PiCdc14 and PiSDA1) and MAPK pathway genes. Our findings provide new insights into catalase functions in eukaryotic pathogens.

miR159 Represses a Constitutive Pathogen Defense Response in Tobacco

MicroR159 (miR159) regulation of GAMYB expression is highly conserved in terrestrial plants; however, its functional role remains poorly understood. In Arabidopsis (Arabidopsis thaliana), although GAMYB-like genes are constitutively transcribed during vegetative growth, their effects are suppressed by strong and constitutive silencing by miR159. GAMYB expression occurs only if miR159 function is inhibited, which results in detrimental pleiotropic defects, questioning the purpose of the miR159-GAMYB pathway. Here, miR159 function was inhibited in tobacco (Nicotiana tabacum) and rice (Oryza sativa) using miRNA MIM159 technology. Similar to observations in Arabidopsis, inhibition of miR159 in tobacco and rice resulted in pleiotropic defects including stunted growth, implying functional conservation of the miR159-GAMYB pathway among angiosperms. In MIM159 tobacco, transcriptome profiling revealed that genes associated with defense and programmed cell death were strongly activated, including a suite of 22 PATHOGENESIS-RELATED PROTEIN (PR) genes that were 100- to 1,000-fold upregulated. Constitutive expression of a miR159-resistant GAMYB transgene in tobacco resulted in phenotypes similar to that of MIM159 tobacco and activated PR gene expression, verifying the dependence of the above-mentioned changes on GAMYB expression. Consistent with the broad defense response, MIM159 tobacco appeared immune to Phytophthora infection. These findings suggest that the tobacco miR159-GAMYB pathway functions in the biotic defense response, which becomes activated upon miR159 inhibition. However, PR gene expression was not upregulated in Arabidopsis or rice when miR159 was inhibited, suggesting that miR159-GAMYB pathway functional differences exist between species, or factors in addition to miR159 inhibition are required in Arabidopsis and rice to activate this broad defense response.

Identification of Pepper CaSBP08 Gene in Defense Response Against Phytophthora capsici Infection

Little information is available on the role of Squamosa promoter binding protein (SBP)-box genes in pepper plants. This family of genes is known to have transcription characteristics specific to plants and to regulate plant growth, development, stress responses, and signal transduction. To investigate their specific effects in pepper (Capsicum annuum), we screened pepper SBP-box family genes (CaSBP genes) for Phytophthora capsici (P. capsici) resistance genes using virus-induced gene silencing. CaSBP08, CaSBP11, CaSBP12, and CaSBP13, which are associated with plant defense responses against P. capsici, were obtained from among fifteen identified CaSBP genes. The function of CaSBP08 was identified in pepper defense response against P. capsici infection in particular. CaSBP08 protein was localized to the nucleus. Silencing of CaSBP08 enhanced resistance to P. capsici infection. Following P. capsici inoculation, the malondialdehyde content, peroxidase activity, and disease index percentage of the CaSBP08-silenced plants decreased compared to the control. Additionally, the expression levels of other defense-related genes, especially those of CaBPR1 and CaSAR8.2, were more strongly induced in CaSBP08-silenced plants than in the control. However, CaSBP08 overexpression in Nicotiana benthamiana enhanced susceptibility to P. capsici infection. This work provides a foundation for the further research on the role of CaSBP genes in plant defense responses against P. capsici infection.

Sodium alginate potentiates antioxidant defense and PR proteins against early blight disease caused by Alternaria solani in Solanum lycopersicum Linn

The use of biopolymers as elicitors in controlling plant diseases is gaining momentum world-wide due to their eco-friendly and non-toxic nature. In the present study, we have used an algal biopolymer (sodium alginate) and tested its applicability as an elicitor in inducing resistance factors against Alternaria solani, which causes early blight disease in Solanum lycopersicum (tomato plant). We have pre-treated tomato plants with different concentrations of sodium alginate (0.2%, 0.4%, and 0.6%) before A. solani infection. We found that sodium alginate has effectively controlled the growth of A. solani. In addition, a significant increase in the expression levels of SOD was observed in response to pathogen infection. The increased protease inhibitors activity further suggest that sodium alginate restrict the development of A. solani infection symptoms in tomato leaves. This corroborates well with the cell death analysis wherein increased sodium alginate pre-treatment results in decreased cell death. Also, the expression profile analyses reveal the induction of genes only in sodium alginate-pretreated tomato leaves, which are implicated in plant defense mechanism. Taken together, our results suggest that sodium alginate can be used as an elicitor to induce resistance against A. solani in tomato plants.

Ectopic Expression of JcWRKY Confers Enhanced Resistance in Transgenic Tobacco Against Macrophomina phaseolina

Plants possess an innate immune system comprising of a complex network of closely regulated defense responses involving differential gene expression mediated by transcription factors (TFs). The WRKYs comprise of an important plant-specific TF family, which is involved in regulation of biotic and abiotic defenses. The overexpression of JcWRKY resulted in improved resistance in transgenic tobacco against Macrophomina phaseolina. The production of reactive oxygen species (ROS) and its detoxification through antioxidative system in the transgenics facilitates defense against Macrophomina. The enhanced catalase activity on Macrophomina infection limits the spread of infection. The transcript expression of antioxidative enzymes gene (CAT and SOD) and salicylic acid (SA) biosynthetic gene ICS1 showed upregulation during Macrophomina infection and combinatorial stress. The enhanced transcript of pathogenesis-related genes PR-1 indicates the accumulation of SA during different stresses. The PR-2 and PR-5 highlight the activation of defense responses comprising of activation of hydrolytic cleavage of glucanases and thaumatin-like proteins causing disruption of fungal cells. The ROS homeostasis in coordination with signaling molecules regulate the defense responses and inhibit fungal growth.

Phytophthora cinnamomi

Phytophthora cinnamomi is one of the most devastating plant pathogens in the world. It infects close to 5000 species of plants, including many of importance in agriculture, forestry and horticulture. The inadvertent introduction of P. cinnamomi into natural ecosystems, including a number of recognized Global Biodiversity Hotspots, has had disastrous consequences for the environment and the biodiversity of flora and fauna. The genus Phytophthora belongs to the Class Oomycetes, a group of fungus-like organisms that initiate plant disease through the production of motile zoospores. Disease control is difficult in agricultural and forestry situations and even more challenging in natural ecosystems as a result of the scale of the problem and the limited range of effective chemical inhibitors. The development of sustainable control measures for the future management of P. cinnamomi requires a comprehensive understanding of the cellular and molecular basis of pathogen development and pathogenicity. The application of next-generation sequencing technologies to generate genomic and transcriptomic data promises to underpin a new era in P. cinnamomi research and discovery. The aim of this review is to integrate bioinformatic analyses of P. cinnamomi sequence data with current knowledge of the cellular and molecular basis of P. cinnamomi growth, development and plant infection. The goal is to provide a framework for future research by highlighting potential pathogenicity genes, shedding light on their possible functions and identifying suitable targets for future control measures.

TAXONOMY

Phytophthora cinnamomi Rands; Kingdom Chromista; Phylum Oomycota or Pseudofungi; Class Oomycetes; Order Peronosporales; Family Peronosporaceae; genus Phytophthora.

HOST RANGE

Infects about 5000 species of plants, including 4000 Australian native species. Host plants important for agriculture and forestry include avocado, chestnut, macadamia, oak, peach and pineapple.

DISEASE SYMPTOMS

A root pathogen which causes rotting of fine and fibrous roots, but which can also cause stem cankers. Root damage may inhibit water movement from roots to shoots, leading to dieback of young shoots. USEFUL WEBSITES: http://fungidb.org/fungidb/; http://genome.jgi.doe.gov/Phyci1/Phyci1.home.html; http://www.ncbi.nlm.nih.gov/assembly/GCA_001314365.1; http://www.ncbi.nlm.nih.gov/assembly/GCA_001314505.1.

Differential Expression Patterns of Pleurotus ostreatus Catalase Genes during Developmental Stages and under Heat Stress

Catalases are ubiquitous hydrogen peroxide-detoxifying enzymes. They participate in fungal growth and development, such as mycelial growth and cellular differentiation, and in protecting fungi from oxidative damage under stressful conditions. To investigate the potential functions of catalases in Pleurotus ostreatus, we obtained two catalase genes from a draft genome sequence of P. ostreatus, and cloned and characterized them (Po-cat1 and Po-cat2). Po-cat1 (group II) and Po-cat2 (group III) encoded putative peptides of 745 and 528 amino acids, respectively. Furthermore, the gene structures were variant between Po-cat1 and Po-cat2. Further research revealed that these two catalase genes have divergent expression patterns during different developmental stages. Po-cat1/Po-cat1 was at a barely detectable level in mycelia, accumulated gradually during reproductive growth, and was maximal in separated spores. But no catalase activity of Po-cat1 was detected by native-PAGE during any part of the developmental stages. In contrast, high Po-cat2/Po-cat2 expression and Po-cat2 activity found in mycelia were gradually lost during reproductive growth, and at a minimal level in separated spores. In addition, these two genes responded differentially under 32 °C and 40 °C heat stresses. Po-cat1 was up-regulated under both temperature conditions, while Po-cat2 was up-regulated at 32 °C but down-regulated at 40 °C. The accumulation of catalase proteins correlated with gene expression. These results indicate that the two divergent catalases in P. ostreatus may play different roles during development and under heat stress.

Identification and characterization of a putative protein disulfide isomerase (HsPDI) as an alleged effector of Heterodera schachtii

The plant-parasitic nematode Heterodera schachtii is an obligate biotroph that induces syncytial feeding sites in roots of its hosts. Nematodes produce effectors that are secreted into the host and facilitate infection process. Here we identified H. schachtii protein disulphide isomerase (HsPDI) as a putative effector that interferes with the host’s redox status. In situ hybridization showed that HsPdi is specifically localized within esophageal glands of pre-parasitic second stage juveniles (J2). HsPdi is up-regulated in the early parasitic J2s. Silencing of HsPdi by RNA interference in the J2s hampers their development and leads to structural malfunctions in associated feeding sites induced in Arabidopsis roots. Expression of HsPDI in Arabidopsis increases plant’s susceptibility towards H. schachtii. HsPdi expression is up-regulated in the presence of exogenous H₂O₂, whereas HsPdi silencing results in increased mortality under H₂O₂ stress. Stable expression of HsPDI in Arabidopsis plants decreases ROS burst induced by flg22. Transiently expressed HsPDI in N. benthamiana leaves is localized in the apoplast. HsPDI plays an important role in the interaction between nematode and plant, probably through inducing local changes in the redox status of infected host tissue. It also contributes to protect the nematode from exogenous H₂O₂ stress.

Lifestyle, gene gain and loss, and transcriptional remodeling cause divergence in the transcriptomes of Phytophthora infestans and Pythium ultimum during potato tuber colonization

BACKGROUND

How pathogen genomes evolve to support distinct lifestyles is not well-understood. The oomycete Phytophthora infestans, the potato blight agent, is a largely biotrophic pathogen that feeds from living host cells, which become necrotic only late in infection. The related oomycete Pythium ultimum grows saprophytically in soil and as a necrotroph in plants, causing massive tissue destruction. To learn what distinguishes their lifestyles, we compared their gene contents and expression patterns in media and a shared host, potato tuber.

RESULTS

Genes related to pathogenesis varied in temporal expression pattern, mRNA level, and family size between the species. A family's aggregate expression during infection was not proportional to size due to transcriptional remodeling and pseudogenization. Ph. infestans had more stage-specific genes, while Py. ultimum tended towards more constitutive expression. Ph. infestans expressed more genes encoding secreted cell wall-degrading enzymes, but other categories such as secreted proteases and ABC transporters had higher transcript levels in Py. ultimum. Species-specific genes were identified including new Pythium genes, perforins, which may disrupt plant membranes. Genome-wide ortholog analyses identified substantial diversified expression, which correlated with sequence divergence. Pseudogenization was associated with gene family expansion, especially in gene clusters.

CONCLUSION

This first large-scale analysis of transcriptional divergence within oomycetes revealed major shifts in genome composition and expression, including subfunctionalization within gene families. Biotrophy and necrotrophy seem determined by species-specific genes and the varied expression of shared pathogenicity factors, which may be useful targets for crop protection.

RNA-Seq Analysis of the Expression of Genes Encoding Cell Wall Degrading Enzymes during Infection of Lupin (Lupinus angustifolius) by Phytophthora parasitica

RNA-Seq analysis has shown that over 60% (12,962) of the predicted transcripts in the Phytophthora parasitica genome are expressed during the first 60 h of lupin root infection. The infection transcriptomes included 278 of the 431 genes encoding P. parasitica cell wall degrading enzymes. The transcriptome data provide strong evidence of global transcriptional cascades of genes whose encoded proteins target the main categories of plant cell wall components. A major cohort of pectinases is predominantly expressed early but as infection progresses, the transcriptome becomes increasingly dominated by transcripts encoding cellulases, hemicellulases, β-1,3-glucanases and glycoproteins. The most highly expressed P. parasitica carbohydrate active enzyme gene contains two CBM1 cellulose binding modules and no catalytic domains. The top 200 differentially expressed genes include β-1,4-glucosidases, β-1,4-glucanases, β-1,4-galactanases, a β-1,3-glucanase, an α-1,4-polygalacturonase, a pectin deacetylase and a pectin methylesterase. Detailed analysis of gene expression profiles provides clues as to the order in which linkages within the complex carbohydrates may come under attack. The gene expression profiles suggest that (i) demethylation of pectic homogalacturonan occurs before its deacetylation; (ii) cleavage of the backbone of pectic rhamnogalacturonan I precedes digestion of its side chains; (iii) early attack on cellulose microfibrils by non-catalytic cellulose-binding proteins and enzymes with auxiliary activities may facilitate subsequent attack by glycosyl hydrolases and enzymes containing CBM1 cellulose-binding modules; (iv) terminal hemicellulose backbone residues are targeted after extensive internal backbone cleavage has occurred; and (v) the carbohydrate chains on glycoproteins are degraded late in infection. A notable feature of the P. parasitica infection transcriptome is the high level of transcription of genes encoding enzymes that degrade β-1,3-glucanases during middle and late stages of infection. The results suggest that high levels of β-1,3-glucanases may effectively degrade callose as it is produced by the plant during the defence response.

Identification and characterization of superoxide dismutase in Phytophthora cinnamomi

Superoxide dismutase (SOD) activities of the oomycete Phytophthora cinnamomi were examined. Five polypeptides with manganese superoxide dismutase (MnSOD) activity were found in mycelium growing in liquid culture with relative molecular weights ranging from approximately 25 to 100 kDa. Comparison with characterized avocado SODs showed no evidence for the presence of either iron or copper/zinc SODs in P. cinnamomi. The level of activity of the MnSOD polypeptides decreased in the presence of avocado root or cell wall components. Growth of P. cinnamomi, measured as dry weight, increased when the mycelium was grown in the presence of superoxide anion (O(2) (-)), which was added exogenously. Our results suggest that the metabolism of O(2) (-) has an important role in the development of P. cinnamomi.

High-level production of human interleukin-10 fusions in tobacco cell suspension cultures

The production of pharmaceutical proteins in plants has made much progress in recent years with the development of transient expression systems, transplastomic technology and humanizing glycosylation patterns in plants. However, the first therapeutic proteins approved for administration to humans and animals were made in plant cell suspensions for reasons of containment, rapid scale-up and lack of toxic contaminants. In this study, we have investigated the production of human interleukin-10 (IL-10) in tobacco BY-2 cell suspension and evaluated the effect of an elastin-like polypeptide tag (ELP) and a green fluorescent protein (GFP) tag on IL-10 accumulation. We report the highest accumulation levels of hIL-10 obtained with any stable plant expression system using the ELP fusion strategy. Although IL-10-ELP has cytokine activity, its activity is reduced compared to unfused IL-10, likely caused by interference of ELP with folding of IL-10. Green fluorescent protein has no effect on IL-10 accumulation, but examining the trafficking of IL-10-GFP over the cell culture cycle revealed fluorescence in the vacuole during the stationary phase of the culture growth cycle. Analysis of isolated vacuoles indicated that GFP alone is found in vacuoles, while the full-size fusion remains in the whole-cell extract. This indicates that GFP is cleaved off prior to its trafficking to the vacuole. On the other hand, IL-10-GFP-ELP remains mostly in the ER and accumulates to high levels. Protein bodies were observed at the end of the culture cycle and are thought to arise as a consequence of high levels of accumulation in the ER.

Molecular evolution of hydrogen peroxide degrading enzymes

For efficient removal of intra- and/or extracellular hydrogen peroxide by dismutation to harmless dioxygen and water (2H(2)O(2) → O(2) + 2H(2)O), nature designed three metalloenzyme families that differ in oligomeric organization, monomer architecture as well as active site geometry and catalytic residues. Here we report on the updated reconstruction of the molecular phylogeny of these three gene families. Ubiquitous typical (monofunctional) heme catalases are found in all domains of life showing a high structural conservation. Their evolution was directed from large subunit towards small subunit proteins and further to fused proteins where the catalase fold was retained but lost its original functionality. Bifunctional catalase-peroxidases were at the origin of one of the two main heme peroxidase superfamilies (i.e. peroxidase-catalase superfamily) and constitute a protein family predominantly present among eubacteria and archaea, but two evolutionary branches are also found in the eukaryotic world. Non-heme manganese catalases are a relatively small protein family with very old roots only present among bacteria and archaea. Phylogenetic analyses of the three protein families reveal features typical (i) for the evolution of whole genomes as well as (ii) for specific evolutionary events including horizontal gene transfer, paralog formation and gene fusion. As catalases have reached a striking diversity among prokaryotic and eukaryotic pathogens, understanding their phylogenetic and molecular relationship and function will contribute to drug design for prevention of diseases of humans, animals and plants.

Catalases play differentiated roles in the adaptation of a fungal entomopathogen to environmental stresses

The catalase family of Beauveria bassiana (fungal entomopathogen) consists of catA (spore-specific), catB (secreted), catP (peroxisomal), catC (cytoplasmic) and catD (secreted peroxidase/catalase), which were distinguished in phylogeny and structure and functionally characterized by constructing single-gene disrupted and rescued mutants for enzymatic and multi-phenotypic analyses. Total catalase activity decreased 89% and 56% in ΔcatB and ΔcatP, corresponding to the losses of upper and lower active bands gel-profiled for all catalases respectively, but only 9-12% in other knockout mutants. Compared with wild type and complement mutants sharing similar enzymatic and phenotypic parameters, all knockout mutants showed significant (9-56%) decreases in the antioxidant capability of their conidia (active ingredients of mycoinsecticides), followed by remarkable phenotypic defects associated with the fungal biocontrol potential. These defects included mainly the losses of 40% thermotolerance (45°C) in ΔcatA, 46-48% UV-B resistance in ΔcatA and ΔcatD, and 33-47% virulence to Spodoptera litura larvae in ΔcatA, ΔcatP and ΔcatD respectively. Moreover, the drastic transcript upregulation of some other catalase genes observed in the normal culture of each knockout mutant revealed functionally complimentary effects among some of the catalase genes, particularly between catB and catC whose knockout mutants displayed little or minor phenotypic changes. However, the five catalase genes functioned redundantly in mediating the fungal tolerance to either hyperosmotic or fungicidal stress. The differentiated roles of five catalases in regulating the B. bassiana virulence and tolerances to oxidative stress, high temperature and UV-B irradiation provide new insights into fungal adaptation to stressful environment and host invasion.

Peroxiredoxins from the plant parasitic root-knot nematode, Meloidogyne incognita, are required for successful development within the host

Root-knot nematodes, Meloidogyne spp., are sedentary biotrophic parasites which are able to infest > 2000 plant species. After root invasion they settle sedentarily inside the vascular cylinder and maintain a compatible interaction for up to 8 weeks. Plant cells respond to pathogen attacks by producing reactive oxygen species (ROS). These ROS, in particular hydroperoxides, are important regulators of host-parasite interactions and partly govern the success or failure of disease. ROS producing and ROS scavenging enzymes from both the pathogen and the host finely tune the redox state at the host-pathogen interface. We have analysed the gene structure and organization of peroxiredoxins (prx) in Meloidogyne incognita and analysed their role in the establishment of the nematode in its host. Meloidogyne incognita has seven prx genes that can be grouped with other nematode prx into three clades. Clade B prx genes are more actively transcribed in parasitic stages compared with free-living pre-parasitic juveniles. We confirmed in vitro the activity of one of these, Mi-prx2.1, on hydrogen peroxide and butylhydroperoxide. We showed by ultrastructural immunocytochemistry the expression of clade B PRX proteins in the hypodermis and pseudocoelum beneath the tissues directly in contact with the environment, both in free-living and parasitic stages. Finally, knock-down of clade B prx genes led to a significant reduction in the ability of the nematodes to complete their life cycle in the host. The expression of clade B PRX proteins in the tissues in close contact with plant cells during parasitism and the impaired development of nematodes inside the host after clade B prx knock-down suggest an important role for these genes during infection.

Phytophthora nicotianae transformants lacking dynein light chain 1 produce non-flagellate zoospores

Biflagellate zoospores of the highly destructive plant pathogens in the genus Phytophthora are responsible for the initiation of infection of host plants. Zoospore motility is a critical component of the infection process because it allows zoospores to actively target suitable infection sites on potential hosts. Flagellar assembly and function in eukaryotes depends on a number of dynein-based molecular motors that facilitate retrograde intraflagellar transport and sliding of adjacent microtubule doublets in the flagellar axonemes. Dynein light chain 1 (DLC1) is one of a number of proteins in the dynein outer arm multiprotein complex. It is a 22 kDa leucine-rich repeat protein that binds to the catalytic motor domain of the dynein gamma heavy chain. We report the cloning and characterization of DLC1 homologues in Phytophthora cinnamomi and Phytophthora nicotianae (PcDLC1 and PnDLC1). PcDLC1 and PnDLC1 are single copy genes that are more highly expressed in sporulating hyphae than in vegetative hyphae, zoospores or germinated cysts. Polyclonal antibodies raised against PnDLC1 locallized PnDLC1 along the length of the flagella of P. nicotianae zoospores. RNAi-mediated silencing of PnDLC1 expression yielded transformants that released non-flagellate, non-motile zoospores from their sporangia. Our observations indicate that zoospore motility is not required for zoospore release from P. nicotianae sporangia or for breakage of the evanescent vesicle into which zoospores are initially discharged.

Effects of catalase on the accumulation of H(2)O(2) in rice cells inoculated with rice blast fungus, Magnaporthe oryzae

Roles of H(2)O(2) in the infection process of Magnaporthe oryzae on rice were investigated. In a leaf sheath assay for up to 48 h post-inoculation, the absence or presence of catalase in the conidia suspension was correlated with the level of accumulated H(2)O(2) in infected leaf cells, as observed by staining with 3',3-diaminobenzidine tetrahydrochloride. In the incompatible interaction, the appearance of autofluorescence or frequency of cell death characterized by granulation (symptoms characteristic of hypersensitive responses) was not significantly affected by the presence of catalase in the conidia suspension. In the leaf blade assay, inoculation of compatible conidia in the presence of catalase produced more severe symptoms than that of conidia in the absence of catalase at 6 days post-inoculation. These results suggest that, in this host-parasite interaction, the primary role of host-produced H(2)O(2) is in limiting hyphal growth after penetration through toxic action. Furthermore, in incompatible interactions, H(2)O(2) is implied not to be a major mediator of hypersensitive cell death.

Different involvement of the mitochondrial, plastidial and cytosolic ascorbate-glutathione redox enzymes in heat shock responses

Plant survival under heat stress requires the activation of proper defence mechanisms to avoid the impairment of metabolic functions. Heat stress leads to the overproduction of reactive oxygen species (ROS) in the cell. In plants, the ascorbate (ASC)-GSH cycle plays a pivotal role in controlling ROS levels and cellular redox homeostasis. Ascorbate peroxidase (APX) is the enzyme of this cycle mainly involved in ROS detoxification. In this study, the ASC-GSH cycle enzymes were analysed in the cytosol, mitochondria and plastids of tobacco Bright Yellow-2 cultured cells. The cells were also subjected to two different heat shocks (HSs; 35 or 55 degrees C for 10 min) and the cell compartments were isolated in both conditions. The results reported here indicate that moderate HS (35 degrees C) does not affect cell viability, whereas cell exposure to 55 degrees C HS induces programmed cell death (PCD). In relation to ASC-GSH cycle, the three analysed compartments have specific enzymatic profiles that are diversely altered by the HS treatments. The cytosol contains the highest activity of all ASC-GSH cycle enzymes and the data reported here suggest that it acts as a redox buffer for the whole cells. In particular, the cytosolic APX seems to be the most versatile enzyme, being its activity enhanced after moderate HS and reduced during PCD induction, whereas the other APX isoenzymes are only affected in the cells undergoing PCD. The relevance of the changes in the different ASC-GSH cycle isoenzymes in allowing cell survival or promoting PCD is discussed.